Optimized tube bundle configuration for controlling a heat exchanger wall temperature

ABSTRACT

A heat exchanger comprises: a first medium inlet operable to receive a first medium into the heat exchanger; a first medium outlet operable to expel the first medium from the heat exchanger; a second medium inlet operable to receive a second medium into the heat exchanger; a second medium outlet operable to expel the second medium from the heat exchanger; and a plurality of parallel tubes running from the second medium inlet to the second medium outlet operable to carry the second medium from the second medium inlet to the second medium outlet, the plurality of tubes including: a first section of a first plurality of tubes, at least one of the first plurality of tubes having a first diameter, situated so that the first medium first flows from the first medium inlet over exterior walls of the first plurality of tubes and through gaps between the first plurality of tubes; and a second section of a second plurality of tubes, at least one of the second plurality of tubes having a second diameter that is different than the first diameter, situated so that after the first medium flows past the first section of the first plurality of tubes the first medium then flows over exterior walls of the second plurality of tubes and through gaps between the second plurality of tubes to the first medium outlet.

GOVERNMENT RIGHTS

This invention was made with Government support under FA8650-07-C-2803 awarded by the United States Air Force. The Government has certain rights in this invention.

FIELD OF INVENTION

The present invention generally relates to heat exchangers, and more specifically relates to heat exchangers having an optimized tube bundle configuration that better controls the heat transfer process between multiple mediums, including better controlling the wall temperature of tubes in the tube bundle that carry a medium participating in a heat exchanging process, as well as the temperature of the medium, in order to prevent overheating of the tube bundle and the medium carried within the tube bundle during the heat exchanging process.

BACKGROUND OF THE INVENTION

Generally, a heat exchanger is a device that may be used for efficient heat transfer between multiple mediums. For example, a heat exchanger may take in a first medium at a low temperature and a second medium at a high temperature. Within the body of the heat exchanger, the first and second mediums may come into contact, either directly or indirectly via a solid wall. When the two mediums come into contact, the high temperature of the second medium may cause the first medium to raise its temperature, while the low temperature of the first medium may also cause the second medium to lower its temperature. As the first medium and the second medium continue to stay in contact, the temperature of the two mediums may continue to converge until the two mediums are no longer in contact or until the temperatures of the two mediums normalize to the same temperature. In other words, heat is exchanged between the two mediums, thus narrowing the difference in temperature between the two mediums.

In the context of a fuel-air heat exchanger, cool fuel and hot air may flow into the fuel-air heat exchanger. A structure within the fuel-air heat exchanger may carry the cool fuel through the fuel-air heat exchanger and heat may be transferred between the cool fuel and the hot air. However, if too much heat transfer occurs between the cool fuel and the hot air, the walls of the structure carrying the fuel may overheat, thereby resulting in coking or pyrolysis of the fuel, resulting in reduced performance. Such deterioration of the fuel may also lead to solid particles being deposited into the structure, thereby resulting to fuel flow control issues. Further, such deposition of solid particles onto the walls of the structure may also result in a further increase in the wall temperature of the structure carrying the fuel, thus further exacerbating the problems described above.

As can be seen, what is needed are ways to control the level of heat transfer that takes place within a heat exchanger.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a heat exchanger comprises: a first medium inlet operable to receive a first medium into the heat exchanger; a first medium outlet operable to expel the first medium from the heat exchanger; a second medium inlet operable to receive a second medium into the heat exchanger; a second medium outlet operable to expel the second medium from the heat exchanger; and a plurality of parallel tubes running from the second medium inlet to the second medium outlet operable to carry the second medium from the second medium inlet to the second medium outlet, the plurality of tubes including: a first section of a first plurality of tubes, at least one of the first plurality of tubes having a first diameter, situated so that the first medium first flows from the first medium inlet over exterior walls of the first plurality of tubes and through gaps between the first plurality of tubes; and a second section of a second plurality of tubes, at least one of the second plurality of tubes having a second diameter that is different than the first diameter, situated so that after the first medium flows past the first section of the first plurality of tubes the first medium then flows over exterior walls of the second plurality of tubes and through gaps between the second plurality of tubes to the first medium outlet.

In another aspect of the present invention, a heat exchanger, comprises: a first section of a first plurality of tubes capable of carrying a first medium, the first section including a first number of tubes, at least one of the first plurality of tubes having first wall thickness, wherein the first plurality of tubes are situated so that after a second medium flows into the heat exchanger the second medium then flows over exterior walls of the first plurality of tubes and through first gaps between the first plurality of tubes; and a second section of a second plurality of tubes including a second number of tubes that is more than the first number of tubes, at least one of the second plurality of tubes having a second wall thickness that is different than the first wall thickness, wherein the second plurality of tubes are situated so that after the second medium flows past the first section of the first plurality of tubes the second medium then flows over exterior walls of the second plurality of tubes and through second gaps between the second plurality of tubes.

In another aspect of the present invention, a method comprises: passing a first medium through a plurality of tubes; passing a second medium over first exterior walls of and through first gaps between first tubes in a first section of the plurality of tubes, wherein the tubes in the first section of the plurality of tubes have a first diameter; and passing the second medium over second exterior walls of and through second gaps between second tubes in a second section of the plurality of tubes after the second medium exits the first gaps, wherein the tubes in the second section of the plurality of tubes have a second diameter that is different than the first diameter.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an isometric front view of an exterior body of a heat exchanger in accordance with an embodiment of the present invention;

FIG. 2 shows a isometric view of a tube bundle within the interior of the heat exchanger shown in FIG. 1;

FIG. 3 shows a section view through the centerline of the tube bundle shown in FIG. 2; and

FIG. 4 shows a view of the layout of the tube bundle of FIG. 2 within the interior of the heat exchanger shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims.

Various inventive features are described below that can each be used independently of one another or in combination with other features.

Broadly, embodiments of the present invention generally provide heat exchangers having an optimized tube bundle configuration that better controls the heat transfer process between multiple mediums and lowers the heat exchanger wall temperature of the tubes in the tube bundle as well as the temperature of a medium carried within the tube bundle, in order to prevent overheating of the tube bundle and the medium carried within the tube bundle. The tube bundle in accordance with embodiments of the invention may carry a first medium within a heat exchanger and may comprise a plurality of sections of tubes where tubes within a first section of the tube bundle that is first within the tube bundle to transfer heat with a second medium have a larger diameter compared to another section of the tube bundle that transfers heat with the second medium after the first section of the tube bundle has already transferred heat with that second medium. By ordering the tube bundle so that tubes with the largest diameters are the first within the tube bundle to transfer heat with another heat medium and then progressively reducing the diameters of the tubes within each successive section of the tube bundle, the rate of heat transfer between the mediums as well as the temperature of the walls of the tubes and the temperature of the first medium carried within the tubes may be better controlled, thus preventing any overheating of the walls of the tubes of the tube bundle and the second medium carried within the tubes.

Embodiments of the present invention may be advantageously used in any heat exchanging application, such as aircraft, to more precisely control the rate of heat transfer between mediums and to more precisely control the temperatures of the heat exchanging mediums during the heat transfer process.

Although embodiments of the present invention will now be described in the context of fuel and air as the heat exchanging mediums, the described embodiments of the present invention may be applicable to any other suitable forms such as liquids, gases, solids, or any mixture of liquids, gases, and solids. Further, the terms hot and cold used herein may not necessarily denote specific temperatures but may simply denote relative temperatures of the mediums compared to one another. Thus, for example, the hot air may be at a relatively higher temperature compared to the cold fuel and the cold fuel may be at a relatively lower temperature compared to the hot air.

FIG. 1 shows an isometric front view of an exterior body of a heat exchanger in accordance with an exemplary embodiment of the present invention. As shown in FIG. 1, the exterior body 140 of the heat exchanger 100 may comprise a first inlet 105, a first outlet 110, a second inlet 115, and a second outlet 120. The first inlet 105, such as an air inlet, may take in a hot medium, such as hot air from a hot source, such as an aircraft engine, while the second inlet 115, such as a fuel inlet, may take in from a cold source, such as a fuel pump, a cold medium, such as cold fuel. After heat is exchanged between the hot air and the cold fuel, the first outlet 110, such as an air outlet, may expel relatively colder air compared to the hot air while the second outlet 120, such as a fuel outlet, may expel relatively hotter fuel compared to the cold fuel.

FIG. 2 shows a isometic view of a tube bundle 200. FIG. 3 shows a section view of the tube bundle 200 of FIG. 2. The tube bundle 200 may comprise a plurality of tubes. The plurality of tubes within the tube bundle 200 may run parallel to one another to form a rectilinear array of parallel tubes. The plurality of tubes may alternatively also be configured as one or more rows of tubes, one or more columns of tubes, as any polygonal array of tubes, as any circular array of tubes, or any other irregularly-shaped array of tubes depending on the particular heat transfer requirements.

The tube bundle 200 may be situated within the interior of the heat exchanger 100 of FIG. 1 so that a plurality of tubes in the tube bundle 200 runs from the second inlet 115 of the heat exchanger 100 of FIG. 1 to the second outlet 120 of FIG. 1. Fluid that enters the heat exchanger 100 of FIG. 1 through the second inlet 115 may be carried through the tube bundle 200 to the second outlet 120 where it may exit the heat exchanger 100 of FIG. 1.

The tube bundle 200 may also be situated within the interior of the heat exchanger 100 of FIG. 1 such that fluid, such as air, coming into the first inlet 105 of FIG. 1 may flow in a single pass cross flow arrangement substantially orthogonal to and over the walls of the plurality of tubes comprising the tube bundle 200 through the gaps 205 between the plurality of tubes in the tube bundle 200, before that fluid is expelled from the heat exchanger 100 of FIG. 1 via the first outlet 110 of FIG. 1.

As fluid passes over the exterior walls of the plurality of tubes in the tube bundle 200 within the interior of the heat exchanger 100 of FIG. 1, heat may be transferred between hot fluid flowing into the heat exchanger 100 of FIG. 1 at the first inlet 105 of FIG. 1 and cool fluid flowing into the heat exchanger 100 of FIG. 1 at the second inlet 115 of FIG. 1 and carried within the plurality of tubes in the tube bundle 200 so that the temperature of the cool fluid within the plurality of tubes and the temperature of the hot fluid flowing over the outside of the tube bundle 200 may begin to converge. In other words, the hot fluid may raise the temperature of the cool fluid while the cool fluid may lower the temperature of the hot fluid, thus producing cooler fluid at the first outlet 110 compared to the hot fluid at the first inlet 105 and warmer fluid at the second outlet 120 compared to the cool fluid at the second inlet 115.

FIG. 4 shows a view of the layout of the plurality of tubes within the interior of the heat exchanger shown in FIG. 1. As shown in FIG. 4, the plurality of tubes may comprise a rectilinear array of tubes having a plurality of sections of a rectilinear array of tubes.

In accordance with the exemplary embodiment of the present invention shown in FIG. 4, the plurality of sections of the rectilinear array of tubes may comprise a first section 405 of a rectilinear array of tubes having a first diameter, a second section 410 of a rectilinear array of tubes having a second diameter that is smaller than the first diameter, and a third section 415 of a rectilinear array of tubes having a third diameter that is less than the second diameter.

Although the exemplary embodiment of the present invention is described in terms of three sections, the plurality of tubes may also be comprised of any plurality of sections. For example, the plurality of tubes may also comprise any number of sections, such as two sections or four sections. The plurality of sections may each comprise a different number or the same number of tubes within each section of the plurality of sections. Further, each section within the plurality of sections may also comprise a unique rectilinear array of tubes that may each have a different number or the same number of rows and columns as compared to other sections within the plurality of sections. In addition, the spacing of the tubes in a section may differ compared to the spacing of tubes within other sections of the plurality of sections.

In operation, hot air may flow over and through the first section 405 of the array of tubes with the tubes within the first section having a first diameter. The first section 405 of the array of tubes may itself be a rectilinear array of tubes having one or more rows of tubes. The tubes within the array of tubes may be spaced apart from one another, thereby creating gaps between the tubes for the hot air to flow through. The gaps between the tubes may be uniform within the first section of the array of tubes, or may be spaced in a non-uniform manner. Each row of tubes within the first section 405 may also be spaced in an offset manner, so that each row of tubes is slightly offset from a neighboring row of tubes, or may be spaced so that each row of tubes is directly above or below a neighboring row of tubes.

As the hot air passes over the exterior walls of the tubes within the first section 405 and through the first section 405 through the spaces between the tubes in the first section 405, heat transfer between the hot air and the cold fuel carried within the tubes may cause the cold fuel within the tubes to rise in temperature and the hot air to decrease in temperature.

Because the hot air may first contact the cold fuel flowing within the first section 405 of the rectilinear array of tubes before the hot air contacts the cold fuel flowing within the second 410 and third 415 sections of the rectilinear array of tubes, the first section 405 of the rectilinear array of tubes may experience the hot air at its hottest temperature within the heat exchanger before the hot air has had any chance to transfer its heat. Therefore, the first section 405 of the array of tubes may need to be designed to withstand heat transferring between the hottest air and the cold fuel and to ensure that the cold fuel carried within the first section 405 of the array of tubes or the walls of the tubes within the first section 405 of the rectilinear array of tubes do not exceed a maximum fluid temperature above which may cause the heat exchanger to malfunction.

To that end, each tube within the first section 405 of the array of tubes may have a first diameter that is larger than the diameters of tubes within the second 410 and third 415 sections of the rectilinear array of tubes. The larger size of the first diameter may help to prevent the fuel running through the first section 405 of the array of tubes as well as the walls of the tubes within the first section 405 of the array of tubes from overheating due to the following reasons, although the present invention is not intended to be limited to the following:

1) the larger inner diameter of the tubes having the first diameter may allow for fuel to flow through the tubes such that the fuel exiting the tubes may have a lower exit temperature as compared to fuels exiting tubes having smaller diameters, such as due to the fuel having a greater flow rate within the tubes having the first diameter as compared to fuel flowing through tubes having smaller diameters;

2) the larger tube diameter and greater spacing between the tubes may result in a lesser air-side heat transfer coefficient compared to the tubes having smaller diameters. Therefore, less heat is transferred from the hot air to the cold fuel; and

3) the greater fuel mass flow through the tubes due to the larger flow area and lower pressure drop characteristics of the larger tube diameter may result in a greater fuel-side heat transfer coefficient compared to the tubes having smaller diameters. Therefore, more of the cool temperature from the cold fuel is transferred to cool the tube walls.

After the hot air passes through the first section 405, the hot air may then proceed to flow over and through a second section 410 of an array of tubes with the tubes within the second section 410 having a second diameter that is less than the first diameter. The second section 410 of the array of tubes may itself be a rectilinear array of tubes having one or more rows of tubes, where the tubes may be spaced apart from each other, thereby creating gaps between the tubes for the hot air to flow through. The gaps between the tubes may be uniform within the first section of the array of tubes, or may be spaced in a non-uniform manner. Each row of tubes within the second section 410 may also be spaced in an offset manner, so that each row of tubes is slightly offset from a neighboring row of tubes, or may be spaced so that each row of tubes is directly above or below a neighboring row of tubes.

Because the hot air passing over the exterior walls of the tubes within the second section 410 has been cooled previously due to the heat transfer action between the hot air and the cold fuel carried within the tubes of the first section 405, the hot air passing over and through the second section 410 of tubes may be cooler than the hot air initially passing through the first section 405 of tubes. Therefore, the second diameter of the tubes within the second section of the rectilinear array of tubes may not need to be as large as the first diameter while still preventing the fuel running through tubes within the second section 410 from overheating or preventing the wall from reaching too high of a temperature.

The array of tubes may further comprise a third section 415 of an array of tubes situated so that hot air may proceed to flow over and thru the third section 415 of tubes after the hot air passes through the second section 410 of tubes. The third section 415 of the array of tubes may itself be a rectilinear array of tubes having one or more rows of tubes, where the tubes may be spaced apart from each other, thereby creating gaps between the tubes for the hot air to flow through. The gaps between the tubes may be uniform within the first section of the array of tubes, or may be spaced in a non-uniform manner. Each row of tubes within the third section 415 may also be spaced in an offset manner, so that each row of tubes is slightly offset from a neighboring row of tubes, or may be spaced so that each row of tubes is directly above or below a neighboring row of tubes.

Applying the principles articulated above, because the hot air that exits the second section 410 of tubes and then flows over and through the third section 415 of tubes may be cooler than the hot air that exits the first section 405 of tubes and then flows over and thru the second section 410 of tubes, the tubes within the third section 415 of tubes may each have a third diameter that may be smaller than the second diameter of the tubes in the second section 410 while still preventing the fuel running through the tubes within the third section 415 from overheating.

After the hot air passes through the third section 415 of tubes, the hot air may be cooler than the hot air that initially passes into the heat exchanger due to the heat transfer action between the hot air and the cold fuel carried within the array of tubes. The resulting cooler hot air may then flow out of the heat exchanger through the air outlet and may then be used by any other components that may require air at a lower temperature than the initial temperature of the hot air passing into the heat exchanger.

The specific dimensions of the first diameter, the second diameter, and the third diameter may depend upon the specific wall temperature of the tubes that one may wish to achieve. In accordance with an exemplary embodiment of the present invention, the first section 405 of the array of tubes may comprise approximately 15% of the total number of tubes, with each of the tubes within the first section 405 having a first wall thickness. The second section 410 of the array of tubes may comprise approximately 25% of the total number of tubes, each of the tubes within the second section 410 having a second wall thickness. The third section 415 of the array of tubes may comprise approximately 60% of the total number of tubes, each of the tubes within the third section 415 each having a third wall thickness. In accordance with an exemplary embodiment of the present invention, the second wall thickness may be larger or smaller than the first wall thickness, and the third wall thickness may be larger or smaller than the first and second wall thicknesses.

In accordance with an exemplary embodiment of the present invention, because the tubes within the first section 405 of the array of tubes may have a diameter that is larger or smaller than the diameter of the tubes within the second section 410 of the array of tubes, the flow rate of fuel within the first section 405 of the array of tubes may correspondingly be greater than the flow rate of fuel within the second section 410 of the array of tubes. Because of its higher flow rate of fuel, the fuel carried within the first section 405 of the array of tubes may also be able to absorb more heat while limiting any temperature increases of the fuel carried within the first section 405 of the array of tubes as compared to the fuel carried within the second section 410 of the array of tubes.

Similarly, because the tubes within the second section 410 of the array of tubes may have a diameter that is larger or smaller than the diameter of the tubes within the third section 415 of the array of tubes, the flow rate of fuel within the second section 410 of the array of tubes may also be correspondingly greater than the flow rate of fuel within the third section 415 of the array of tubes. Because of its higher flow rate of fuel, the fuel carried within the second section 410 of the array of tubes may also be able to absorb more heat while limiting any temperature increases of the fuel carried within the second section 410 of the array of tubes as compared to the fuel carried within the third section 415 of the array of tubes.

In accordance with an exemplary embodiment of the present invention, because the tubes within the first section 405 of the array of tubes may have a different tube spacing and configuration than the tube spacing and configuration within the second section 410 of the array of tubes, the heat transfer coefficient of the flow over the tubes within the first section 405 of the array of tubes may correspondingly be less than the heat transfer coefficient of flow over the tubes within the second section 410 of the array of tubes. Because of this lower heat transfer coefficient for the flow over the tubes in the first section 405, the tubes in the first section 405 may be able to tolerate greater inlet temperature and still limit any temperature increases of the fuel carried within the first section 405 of the array of tubes as compared with the fuel carried within the second section 410 of the array of tubes. Also, the temperature of the tube wall of the first section 405 may also be limited as compared to the tube wall of the second section 410.

Similarly, because the tubes within the second section 410 of the array of tubes may have a different tube spacing and configuration than the tube spacing and configuration within the third section 415 of the array of tubes, the heat transfer coefficient of the flow over the tubes within the second section 410 of the array of tubes may correspondingly be less than the heat transfer coefficient of flow over the tubes within the third section 415 of the array of tubes. Because of this lower heat transfer coefficient for the flow over the tubes in the second section 410, the tubes in the second section 410 may be able to tolerate higher inlet temperature and still limit any temperature increases of the fuel carried within the second section 410 of the array of tubes as compared with the fuel carried within the third section 415 of the array of tubes. Also, the temperature of the tube wall of the second section 410 may also be limited as compared to the tube wall of the third section 415.

When the temperature difference between the temperature of the hot air and the temperature of the cold fuel is close to zero, meaning that there may be little heat transfer occurring within the heat exchanger, the array of tubes may be arranged so that the flow split of the cold fuel may be approximately 40% for the first section 405 of the array of tubes, approximately 34% for the second section 410 of the array of tubes, and approximately 26% for the third section 415 of the array of tubes.

When the differences in temperature between the hot air and the cold fuel becomes larger, which may mean that heat transfer is taking place between the hot air and the cold fuel within the heat exchanger, the array of tubes may be arranged so that the flow split of the cold fuel may become approximately 36% for the first section 405 of the array of tubes, approximately 34% for the second section 410 of the array of tubes, and approximately 30% for the third section 415 of the array of tubes. Such a change in the fuel flow split may be caused by the fact that heat may be transferred at a greater rate at the third section of the rectilinear array of tubes compared to the first and second sections because the tubes in the third section may have a smaller diameter compared with the tubes within the first and second sections, thus causing fuel flowing through the third section to increase its flow rate at a greater rate compared with the fuel flowing through the first and second sections.

Although exemplary embodiments of the present invention have been described above in terms of a heat exchanger having a rectilinear array of tubes, alternative exemplary embodiments of the present invention may also include any other heat exchanging apparatus, such as a shell and tube heat exchanger, having an arrangement of tubes that may not be a rectilinear array, so long as the tubes within these alternative heat exchanging apparatuses are arranged according to at least some of the examples described above, such as having one or more sections of tubes with differing diameters and/or differing wall thicknesses, such as having one or more sections of tubes with differing tube spacing, and such as using other augmentation devices instead of tubes, such as to better control the heat exchanging process occurring within the heat exchanging processes.

Although embodiments of the present invention are described in terms of a single pass cross flow arrangement, other embodiments of the present invention may include designs using multiple pass cross flow arrangements, with the multiple passes being present on the fluid that flows over the tubes side or with the multiple passes being present on the fluid flowing within the tubes.

It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims. 

1. A heat exchanger comprising: a first medium inlet operable to receive a first medium into the heat exchanger; a first medium outlet operable to expel the first medium from the heat exchanger; a second medium inlet operable to receive a second medium into the heat exchanger; a second medium outlet operable to expel the second medium from the heat exchanger; and a plurality of parallel tubes running from the second medium inlet to the second medium outlet operable to carry the second medium from the second medium inlet to the second medium outlet, the plurality of tubes including: a first section of a first plurality of tubes, at least one of the first plurality of tubes having a first diameter, situated so that the first medium first flows from the first medium inlet over exterior walls of the first plurality of tubes and through gaps between the first plurality of tubes; and a second section of a second plurality of tubes, at least one of the second plurality of tubes having a second diameter that is different than the first diameter, situated so that after the first medium flows past the first section of the first plurality of tubes the first medium then flows over exterior walls of the second plurality of tubes and through gaps between the second plurality of tubes to the first medium outlet.
 2. The heat exchanger of claim 1, wherein: the first section of the plurality of tubes have a first tube spacing and arrangement; and the second section of the second plurality of tubes have a second tube spacing and arrangement that is different than the first section.
 3. The heat exchanger of claim 1, wherein the plurality of parallel tubes further comprises: a third section of a third plurality of tubes each having a third diameter that is different than the second diameter, situated so that after the first medium flows past the second section of the second plurality of tubes the first medium then flows over exterior walls of the third section of the plurality of tubes and through gaps between the third plurality of tubes to the first medium outlet.
 4. The heat exchanger of claim 3, wherein: the first section of the first plurality of tubes comprises a first rectilinear array of parallel tubes; the second section of the second plurality of tubes comprises a second rectilinear array of parallel tubes; and the third section of the third plurality of tubes comprises a third rectilinear array of parallel tubes.
 5. The heat exchanger of claim 1, wherein: the first medium flows orthogonal to the plurality of parallel tubes.
 6. The heat exchanger of claim 1, wherein: the first medium is at a first temperature; and the second medium is at a second temperature that is different from the first temperature.
 7. The heat exchanger of claim 6, wherein: the first medium is at a higher temperature compared to the second medium;
 8. The heat exchanger of claim 7, wherein: the first medium flowing out of the first medium outlet is at a lower temperature compared to the first medium flowing into the first medium inlet; and the second medium flowing out of the second medium outlet is at a higher temperature compared to the second medium flowing into the second medium inlet.
 9. The heat exchanger of claim 1, wherein: the second medium flowing through the first plurality of tubes has a first flow rate through the first plurality of tubes that is higher than a second flow rate of the second medium flowing through the second plurality of tubes.
 10. A heat exchanger, comprising: a first section of a first plurality of tubes capable of carrying a first medium, the first section including a first number of tubes, at least one of the first plurality of tubes having first wall thickness, wherein the first plurality of tubes are situated so that after a second medium flows into the heat exchanger the second medium then flows over exterior walls of the first plurality of tubes and through first gaps between the first plurality of tubes; and a second section of a second plurality of tubes including a second number of tubes that is more than the first number of tubes, at least one of the second plurality of tubes having a second wall thickness that is different than the first wall thickness, wherein the second plurality of tubes are situated so that after the second medium flows past the first section of the first plurality of tubes the second medium then flows over exterior walls of the second plurality of tubes and through second gaps between the second plurality of tubes.
 11. The heat exchanger of claim 10, wherein: the first gaps between the first plurality of tubes are a first uniform distance within the first section of the first plurality of tubes; and the second gaps between the second plurality of tubes are a second uniform distance within the second section of the second plurality of tubes.
 12. The heat exchanger of claim 10, wherein: the first section of the first plurality of tubes comprises a first rectilinear array of tubes; and the second section of the second plurality of tubes comprises a second rectilinear array of tubes.
 13. The heat exchanger of claim 10, further comprising: a third section of a third plurality of tubes including a third number of tubes that is more than the second number of tubes, at least one of the third plurality of tubes having a third diameter that is different than the second diameter, wherein the third plurality of tubes are situated so that after the second medium flows past the second section of the second plurality of tubes the second medium then flows over exterior walls of the third plurality of tubes and through third gaps between the third plurality of tubes.
 14. The heat exchanger of claim 10, wherein the second medium is at a higher temperature than the first medium.
 15. The heat exchanger of claim 14, wherein the first medium comprises fuel and the first medium comprises air.
 16. The heat exchanger of claim 10, wherein the first plurality of tubes carries the first medium from a first medium inlet to a first medium outlet within the heat exchanger.
 17. A method comprising: passing a first medium through a plurality of tubes; passing a second medium over first exterior walls of and through first gaps between first tubes in a first section of the plurality of tubes, wherein the tubes in the first section of the plurality of tubes have a first diameter; and passing the second medium over second exterior walls of and through second gaps between second tubes in a second section of the plurality of tubes after the second medium exits the first gaps, wherein the tubes in the second section of the plurality of tubes have a second diameter that is different than the first diameter.
 18. The method of claim 17, further comprising: expelling the second medium through a second outlet after the second medium exits the second gaps; and expelling the first medium through a first outlet.
 19. The method of claim 17, wherein the passing the second medium over the first exterior walls of and through the first gaps between the first tubes in the first section of the plurality of tubes further comprises: exchanging heat between the second medium and first medium carried within the first tubes in the first section of the plurality of tubes.
 20. The method of claim 17, wherein the passing the second medium over the first exterior walls of and through the first gaps between the first tubes in the first section of the plurality of tubes further comprises: passing the second medium orthogonal to the first tubes of the first section of the plurality of tube. 